Generally, an organic light emitting diode (OLED) being a flat panel display includes a plurality of pixels 100 arranged in the form of matrix as shown in FIG. 1, and each of the pixels consists of three unit pixels of an unit pixel 110R to embody a red color (R), an unit pixel 120G to embody a green color (G), and an unit pixel 130B to embody a blue color (B).
The R unit pixel 110R includes a red electroluminescence (EL) device 115 including a red (R) light emitting layer, a driving transistor 113 for supplying a current to the red EL device 115, and a switching transistor 111 for switching the current supply from the driving transistor 113 to the red EL device 115.
The G unit pixel 120G includes a green EL device 125 including a green (G) light emitting layer, a driving transistor 123 for supplying a current to the green EL device 125, and a switching transistor 121 for switching the current supply from the driving transistor 123 to the green EL device 125.
The B unit pixel 130B includes a blue EL device 135 including a blue (B) light emitting layer, a driving transistor 133 for supplying a current to the blue EL device 135, and a switching transistor 131 for switching the current supply from the driving transistor 133 to the blue EL device 135.
Conventionally, the driving transistors 113, 123 and 133 of the R, G and B unit pixels 110R, 120G and 130B of an OLED have the same size, that is, the ratio W/L of the width W to the length L of the channel layer is the same. The order of the EL devices in the order of their luminous efficiency is B, R and G unit pixels. Since the sizes of the driving transistors 113, 123 and 133 of the R, G, and B unit pixels 110R, 120G and 130B are same while luminous efficiencies of each R, G and B EL layer 115, 125 and 135 are different from one another, it was difficult to embody the white balance.
In order to embody the white balance, a relatively small quantity of current should be supplied to the EL device having high luminous efficiency, for example, the green EL device, and a relatively large quantity of current should be supplied to the red and blue EL devices having a lower luminous efficiency.
Here, since a current Id flowing to the EL device through the driving transistor begins to flow when the driving transistor is in the saturation state, the current is expressed as follows.Id=Cox μW(Vg−Vth)2/2L  (1)
Therefore, one of the methods for controlling the current flowing to the EL device in order to embody the white balance is to make the sizes of the driving transistors of the R, G and B unit pixels, that is, the ratio W/L of the width W to the length L of the channel layer, different and then to control the quantity of the current flowing to the EL devices of the R, G and B unit pixels. A method for controlling the quantity of current flowing to the EL device in accordance with the size of the transistor is disclosed in the Japanese Laid-open Publication No. 2001-109399. In this Japanese patent, the sizes of the driving transistors of the R, G and B unit pixels are formed differently in accordance with the luminous efficiency of the EL device in each R, G and B unit pixel. That is, the quantity of the current flowing to the EL device of the R, G and B unit pixels is controlled by making the size of the driving transistor of the unit pixel to embody the green (G) having a high luminous efficiency smaller than the size of the driving transistors of the unit pixels to embody the red (R) or blue (B) having a relatively low luminous efficiencies.
Another method to embody the white balance is to make the dimensions of the light emitting layers of R, G and B unit pixels different, which is disclosed in the Japanese Laid-open Patent Publication No. 2001-290441. In this Japanese patent, the same luminance is generated from the R, G and B unit pixels by making the light emitting areas different in accordance with the light emitting efficiencies of the EL devices of the R, G and B unit pixels. That is, the same luminance is generated from the R, G and B unit pixels by making light emitting areas of the R unit pixel or B unit pixel having lower luminous efficiencies larger than that of the G unit pixel having high luminous efficiency relatively.
However, in the conventional method for embodying the white balance described above, the light emitting area of the unit pixel having low luminous efficiency among the R, G and B unit pixels is enlarged, or the size of the transistor of the unit pixel having low luminous efficiency among the R, G and B unit pixels is increased. Therefore, the area occupied in each pixel is increased, and it is not easy to apply the method to a high definition display.